This invention pertains generally to electrical safety equipment and more particularly to line isolation monitors for detecting ground faults on power lines normally isolated from ground.
Isolated power systems have been used for many years in hospital operating rooms to reduce the danger of sparks to ground which might ignite the anesthetic used therein. Recently, so called safe power centers have been utilized in critical care areas and coronary care units. Generally, isolated power systems and safe power systems have isolation transformers which deliver AC power to conductors which are electrically isolated from the grounded power system which supplies the hospital. The isolated lines are commonly monitored to make certain that they remain isolated from ground.
Early isolation monitors, such as differential transformer systems, were not able to detect balanced faults existing simultaneously from both sides of the isolated power system to ground. More recently, sampling systems which alternately monitor the current between two points and ground have been used. While the sampling systems can detect balanced faults, they produce switching transients which interfere with the operation of various patient monitoring devices. The problem is particularly serious in the case of heart rate monitors since a line isolation monitor operating at the common sampling rate of 1.5 Hz produces transient spikes at the rate of 180 per minute, and this falls in the normal range of heart rate for a baby. The problem is so severe that hospital personnel have intentionally disabled line isolation monitors to prevent such interference.
Sampling line isolation monitors heretofore provided have generally utilized relays to perform the sampling. Attempts have been made without success to suppress the switching transients with capacitors, resistors, and diodes. It has been found that most of the transients can be eliminated by increasing the impedance of the monitor to reduce the current which is switched. However, this requires an amplifier or a more sensitive meter and recalibration of the system. Attempts to utilize zero-point switching have been unsuccessful due to the lack of a suitable phase reference.